It looks like you have problem with your depth stencil state. If you are using SpriteFonts they might change your render states so you need to reset them before 3D rendering. Try to put the code below just before your rendering.

You might have different render states for different models. So you have to change them depending on what you are trying to do. But for the normal 3D rendering default settings should suffice. If all your models are using default render states you just need to set them once in the beginning of your application. But as I said SpriteBatch changes it before it does its 2D rendering. So if you are using SpriteBatch and some 3D models you need to reset your render states every time before you render your 3D models.